1. Field of the Invention
The invention relates to processes for the production of high strength polyethylene tape articles from high strength ultra-high molecular weight multi-filament yarns, and to the tape articles, fabrics, laminates and impact resistant materials made therefrom.
2. Description of the Related Art
Impact resistant and penetration resistant materials find uses in many applications such as sports equipment, safety garments, and most critically, in personal body armor. The construction of body armor for personal protection is an ancient but not archaic art. Metal armor, already well known to the Egyptians by 1500 B.C., persisted in use until about the end of the 17th century. In the current era, body armor has again become practical through the discovery of new strong fibers such as aramids, ultra-high molecular weight polyethylene (UHMW PE), and polybenzazoles.
Various fiber-reinforced constructions are known for use in impact-resistant, ballistic-resistant and penetration-resistant articles such as helmets, panels, and vests. These articles display varying degrees of resistance to penetration by impact from projectiles or knives, and have varying degrees of effectiveness per unit of weight. A measure of the ballistic-resistance efficiency is the energy removed from a projectile per unit of the target's areal density. This is known as the Specific Energy Absorption, abbreviated as “SEA”, and having units of Joules per Kg/m2 or J-m2/Kg.
The SEA of a fibrous construction is known to generally increase with increasing strength, tensile modulus and energy-to-break of the constituent fibers. However, other factors, such as the shape of the fibrous reinforcement, may come into play. U.S. Pat. No. 4,623,574, presents a comparison between the ballistic effectiveness of a composite constructed with a ribbon-shaped reinforcement versus one using a multi-filament yarn: both of UHMW PE. The fiber had a higher tenacity than the ribbon: 30 grams/denier (abbreviated g/d) versus 23.6 g/d. Nevertheless, the SEA of the composite constructed with the ribbon was somewhat higher than the SEA of the composite constructed with the yarn. U.S. Pat. No. 4,623,574 teaches that elastomer coated strip or ribbon can be more effective than coated-filament yarn in producing ballistic resistant composites.
The preparation of UHMW PE articles having flat cross-sections by a process commonly known as “gel spinning” is described in U.S. Pat. No. 4,413,110. A ribbon prepared by the method of U.S. Pat. No. 4,413,110 is described in U.S. Pat. No. 4,623,574. It had a width of 0.64 cm, a denier of 1240, and a tenacity of 23.9 g/d (corresponding to a tensile strength of 2.04 GPa).
Other processes for the preparation of UHMW PE tape articles are described in U.S. Pat. Nos. 4,996,011; 5,002,714; 5,091,133; 5,106,555, 5,200,129; 5,578,373; 5,628,946; 6,017,834; 6,328,923 B1; 6,458,727B1; 7,279,441 B2; 6,951,685 B1; U.S. Pat. No. 7,470,459 B1; United States Patent Publications 2008/0251960 A1; 2008/0318016 A1; and WO 2009/056286 A1.
In one group of these patents, polyethylene filaments were subjected to a contact pressure at elevated temperature to selectively melt a portion of the fibers to bind the filaments together, followed by compression of the bound fibers. An UHMW PE SPECTRA® yarn subjected to this process in U.S. Pat. No. 5,628,946 lost 69% of its longitudinal modulus.
In another group of these patents, polyethylene powder was compressed at elevated temperature to bond the particles into a continuous sheet that was further compressed and stretched. U.S. Pat. No. 5,091,133 describes a fiber made by this latter process having a tensile strength of 3.4 GPa. Polyethylene tapes so produced are commercially available under the trademark TENSYLON®. The highest tenacity reported on the TENSYLON® web site is 19.5 g/d (tensile strength of 1.67 GPa).
Research publications describing preparation of polyethylene tapes and/or flattening of UHMW PE fibers include the following:    R. J. Van et al., “The Hot Compaction of SPECTRA Gel-Spun Polyethylene Fibre”, J. Matl. Sci., 32, 4821-4831 (1997)    A. Kaito et al., “Hot Rolling and Quench Rolling of Ultrahigh Molecular Weight Polyethylene”, J. Appl. Poly Sci., 29, 1207-1220 (1983); “Preparation of High Modulus Polyethylene Sheet by the Roller-Drawing Method”, J. Appl. Poly Sci., 30, 1241-1255 (1985); “Roller Drawing of Ultrahigh Molecular Weight Polyethylene”, J. Appl. Poly. Sci., 30, 4591-4608 (1985)
The highest breaking strength reported in these publications was approximately 0.65 GPa corresponding to a tenacity of about 7.6 g/d. In the publication by Van et al. cited above, the longitudinal modulus of the UHMW PE was reduced by 27 to 74%.
Each of the patents and publications cited above represents improvement in the state of the art. However, none describes the specific process of this invention and none satisfies all of the needs met by this invention. There is a continuing need for materials that provide superior resistance to penetration by ballistic projectiles. As noted above, the SEA of a fibrous construction is known to generally increase with increasing strength, tensile modulus and energy-to-break of the constituent fibers. Highly oriented UHMW PE multi-filament yarns having strengths much greater than those of the tape articles of the prior art are commercially available. Conversion of such high strength yarns into tape articles with substantial retention of strength could be helpful. It could also be helpful to provide woven and non-woven fabrics and ballistic and penetration resistant articles comprising said tape articles.